1. Field of the Invention
This invention relates to porous fibrous sheets, such as papers and nonwoven fabrics, which comprise nanofibers or a combination of wood pulp and nanofibers.
2. Description of the Related Art
Barrier to microbial penetration is an important and essential property of materials used for packaging medical devices. Materials currently used in medical packaging include a variety of films, flash-spun polyolefin nonwovens, and medical grade papers. In cases where gas or plasma sterilization (e.g., ethylene oxide, Sterrad®, etc.) is used to sterilize the contents of a package, the package generally includes a film, such as a thermoformed film, forming the bottom web that is heat-sealed to a porous and gas permeable lid, such as paper or flash-spun polyolefin sheet. Alternately, the package may be in the form of a pouch comprising a porous layer heat-sealed to a film. The porous lid or layer must allow the sterilant gas or plasma to enter and exit the package to sterilize its contents and at the same time provide a barrier to microbial penetration in order for the medical device to remain sterile until it is used.
The microbial barrier properties of a porous fibrous sheet depend on the average pore size, sheet thickness, size of fibers, fiber morphology, etc. Porous microbial barrier sheets prevent penetration by microbial spores and particles that range in size from sub-micrometer to a few micrometers. The ability of porous sheets to prevent bacterial penetration is measured by their Log Reduction Value (LRV). The higher the LRV value, the better a material is in preventing microbial penetration of the package. For example, the LRV of flash-spun polyolefin sheets used in medical packaging ranges between about 3.2 and 5.5 or higher, as the basis weight (BW) increases from about 1.65 to 2.2 oz/yd2 (55.9 to 74.6 g/m2). Medical grade papers known in the art have LRV's between about 1 and 3, depending on their basis weight, pore size, additive treatments, etc., and are much less effective as microbial barriers than flash-spun materials. Although paper has been improved through many years of use in medical packaging, it still has further limitations in strength, tear resistance and also peelability. Special peelable coatings are used such that they form the weak link in heat sealed packages and tend to fail cohesively when packages are peeled to avoid tearing the paper, which results in linting of the medical device.
Koslow, Patent Application Publication No. U.S. 2003/0177909 describes an air filter medium comprising nanofibers. A coating of nanofibers can be used to enhance the performance of filter media. The nanofibers are preferably fibrillated nanofibers. In one embodiment a filter medium is prepared from a blend of fibrillated nanofibers and glass microfibers.
Generally, increasing the basis weight can increase the barrier properties of nonwoven webs. It would be desirable to improve the barrier properties in a cost-effective manner without increasing basis weight or changing the nonwoven properties that control their porosity and breathability. There remains a need for porous fibrous sheet structures having improved microbial barrier properties for use in medical packaging.